48 shows a continuation of the same spectra into a region where the radiant energy is invisible to the eye, but is capable of being photographed. [Illustration: FIG. 47.--Green and blue part of the spectrum of an electric arc light.] It is only when a lens is placed between the lamp and the slit of the spectroscope that the three spectra are shown distinct from each other as in the figure. The purpose of the lens is to make a picture of the lamp upon the slit, so that all the radiant energy from any one point of the arc may be brought to one part of the slit, and thus appear in the resulting spectrum separated from the energy which comes from every other part of the arc. Such an instrument is called an _analyzing spectroscope_ while one without the lens is called an _integrating spectroscope_, since it furnishes to each point of the slit a sample of the radiant energy coming from every part of the source of light, and thus produces only an average spectrum of that source without distinction of its parts. When a spectroscope is attached to a telescope, as is often done (see Fig. 49), the eyepiece is removed to make way for it, and the telescope objective takes the part of the analyzing lens. A camera is frequently combined with such an apparatus to photograph the spectra it furnishes, and the whole instrument is then called a _spectrograph_. [Illustration: FIG. 48.--Violet and ultraviolet parts of spectrum of an arc lamp.] [Illustration: FIG. 49.--A spectroscope attached to the Yerkes telescope.] 85. SPECTRUM ANALYSIS.--Having seen the mechanism of the spectroscope by which the light incident upon it is resolved into its constituent parts and drawn out into a series of colors arranged in the order of their wave lengths, we have now to consider the interpretation which is to be placed upon the various kinds of spectra which may be seen, and here we rely upon the experience of physicists and chemists, from whom we learn as follows: